Present day telephone systems usually include a switching network for providing communications paths between telephone lines served by the system. Such communications paths are provided under the direction of a call controller in response to signalling received from telephone lines. The communications paths are typically time defined as channels within periodically occurring frames and are usually referred to as time division multiplexed (TDM) channels. TDM channels are usually limited to digital transmission of binary signals across the switching network. Voice communications may be carried on via the TDM channels in the well known pulse code modulated (PCM) signal format. Hence many present day telephone systems are referred to as TDM PCM telephone systems.
Most present day telephone lines are analog in nature, being operable in a signal format within a voice band of frequencies. However, some telephone lines operate in adherence with the recommended integrated services digital network (ISDN) standard signal format. The ISDN signal format requires an operating frequency spectrum which extends many octaves beyond the voice band. Yet other telephone lines operate in proprietary signal formats in association with private branch exchanges (PBXs) and the like. In any case, none of the signal formats of operating telephone lines is likely to be compatible with the digital signal operating format of the typical time division multiplex telephone switching facility. Hence for at least each telephone line in active service, there is provided a line interface circuit which exchanges signals between the active telephone line and a digital signals communications path provided by the digital switching network. The line circuit functions to provide all or most of the interface functions required in order that the switching facility provide telephone service for the telephone line. These functions typically include:
supplying direct current for energizing the telephone line and an associated telephone instrument, PA1 performing two wire to four wire signal conversions, and usually, PA1 converting digital signals, received from the digital signals communications path, to analog signals for receipt by the associated telephone instrument, PA1 converting analog signals, originating at the associated telephone instrument, to digital signals for transmission via the digital signals communications path, PA1 coupling of the transmit signal to and the receive signal from a time defined channel in a TDM signal stream having been designated as the digital signals communications path in the switching network, and PA1 sensing signalling such as for ON HOOK and OFF HOOK states of the telephone line. PA1 m line interface circuits, each of which provides signal translation between an operating signal format of a preassigned one of the m communications paths in the telephone system and a one of the telephone line appearances; PA1 a sensing circuit means connected with each of the telephone line appearances, each sensing circuit means for sensing ON HOOK and OFF HOOK states of a telephone line; PA1 analog switch means including n groups of z crosspoints each of the z crosspoints in a group being operable to connect and disconnect a one of the n telephone line appearances at a corresponding one of the m line interface circuits, the n groups of z crosspoints being so arranged that each of the m line interface circuits is connectable with more than z of the line appearances, and PA1 control means for directing switching operations of the analog switch means to connect a telephone line appearance with a corresponding idle one of the line interface circuits via the associated crosspoint, in response to a transition to an OFF HOOK state having been sensed by the sensing circuit means. PA1 providing connectability between each of the n telephone line appearances and less than all the m communication paths, such that any one of the m communications paths is connectable with any of at least a plurality of the n telephone line appearances; and PA1 in an event of a service request occurring at a one of the n telephone line appearances for which all of the connectable communications paths are already engaged by others of the n telephone line appearances, and in the event that there is an alternately engageable communications path available to one of said others of the n telephone line appearances, transferring the one of said others of the n telephone line appearances to be engaged with the alternate communications path, and thereafter PA1 engaging said one telephone line appearance with the communications path from which the one of said others of the n telephone line appearances is transferred.
One important function of the practical line interface circuit, in cooperation with various protection devices, is that of isolating the switching network, which is somewhat delicate, from transient and potentially catastrophic electrical events to which a telephone line can be exposed, events such as lightning strikes and powerline crosses.
As is well known, the grade or quality of service that any telephone system may provide is related to the peak traffic the system must handle in relation to n telephone line connections or appearances at the switching network. In North America, as a matter of cost, it is considered preferable to concentrate n telephone lines upon m accessible channels in the switching network, where n is in a range of between about 2 m and 6 m. This four to one concentration ratio is based on the statistical fact that in most telephone exchanges very seldom are there as many as 1/4 of the telephones in use at any one time. Coupling the n telephone lines with the m channels is typically achieved in either of two ways.
In one example, each line interface circuit of a group of 24 line interface circuits is connected to operate with one of 24 TDM channels. A switch (n.times.m) matrix is provided to connect up to 24 of as many as 96 or so telephone lines to the 24 TDM channels. As it is rare that more than 24 of the telephone lines in the group will be in an active (OFF HOOK) state at any one time, telephone service is effectively always available to any one of the telephone lines at any time.
In another example, each of up to 120 or so telephone lines is each terminated at a preassigned one of n line interface circuits. Each of the n line interface circuits has common access to m channels such that up to 30 of the telephone lines may be switched to any of the 30 channels at any time, for the duration of a telephone call. As in the first example, this provides a concentration of about 4 to 1. As it is rare that more than 30 of the telephone lines in the group will be in an active (OFF HOOK) state at any one time, telephone service is effectively always available to any one of the telephone lines at any time.
In the first example, economy is achieved in that only m line interface circuits are required to serve four times as many lines. However, this is at the expense of having to provide the analog switch (n.times.m) matrix. In the second example, the cost of the analog (n.times.m) switch matrix is avoided, but at the expense of having to provide n line interface circuits, that is, one in connection with each of the n telephone lines. In typical telephone digital switching systems, the provision of adequate line interfacing apparatus is a major expense.
It is an object of the invention to reduce the cost and complexity of a switch matrix for interconnecting analog telephone lines with line interface circuits.
It is also an object of the invention to provide a method for controlling the switch matrix such that very little, if any, consequent reduction in service availability is experienced by users of a telephone system wherein the switch matrix is used.